In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as, optical, ultrasonic, surface capacitive, projected capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with an optically transparent conductive layer are separated by spacers and face each other. A current is applied to the optically transparent conductive material and the voltage of the glass plate with an optically transparent conductive layer is measured. In contrast, a capacitive touchscreen has a basic configuration in which an optically transparent conductive layer is provided on a base material and there are no movable parts. Capacitive touchscreens are used in various applications due to their high durability and high transmission. Further, projected capacitive technology has an advantage of simultaneous multipoint detection, and therefore is widely used for smartphones, tablet PCs, etc.
As optically transparent conductive material for touchscreens, those having an optically transparent conductive layer made of ITO (indium tin oxide) on a base material have been commonly used. However, since an optically transparent conductive layer made of ITO has high refractive index and high surface light reflectivity, the total light transmittance of an optically transparent conductive material utilizing the optically transparent conductive layer made of ITO is unfavorably low. In addition, due to low flexibility, the optically transparent conductive layer made of ITO is prone to crack when bent, resulting in increased electric resistance of the optically transparent conductive material.
As an alternative to the optically transparent conductive material having an optically transparent conductive layer made of ITO formed on a base material, an optically transparent conductive material obtained by a semi-additive method for forming a metal pattern, the method comprising forming a thin catalyst layer on a base material, forming a resist pattern on the catalyst layer, forming a laminated metal layer in an opening of the resist layer by plating, and finally removing the resist layer and the base metal protected by the resist layer, is disclosed in, for example, Patent Literature 1 and Patent Literature 2.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed and a photosensitive material is used as a precursor to a conductive material has been proposed. For example, Patent Literature 3, Patent Literature 4, and Patent Literature 5 disclose a technology for forming a metal silver pattern by a reaction of a conductive material precursor having a physical development nucleus layer and a silver halide emulsion layer in this order on a base material with a soluble silver halide forming agent and a reducing agent in an alkaline fluid. The patterning by this method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods. An additional advantage is that a silver pattern obtained by this method has a higher flexibility, i.e., a longer flexing life as compared with an optically transparent conductive layer made of ITO.
In the aforementioned projected capacitive touchscreen, two optically transparent conductive materials on each of which a plurality of sensor parts are patterened in the same plane are joined together, and the two serves as a touch sensor. If such a touch sensor is composed of only a plurality of sensors, the sensor part is conspicuous. In an attempt to avoid this, a dummy part that is not electrically connected to the sensor part is arranged in a place other than the sensor part. However, while in operation, an operator of a touchscreen usually keeps staring at the display, and as a result tends to recognize the difference between the sensor part and the dummy part (highly visible), having a feeling of strangeness. In particular, a projected capacitive touchscreen produced with use of an optically transparent conductive material having a metal pattern on its base material markedly has the problem of the visibility of the sensor part and the dummy part.
To address this problem, in Patent Literature 6, a grid-like metal pattern is divided by a slit (belt-like portion without the metallic pattern) into a sensor part and a dummy part, and for the purpose of reducing the visibility, the slit width is in a range from 20 μm to the maximum dimension of the grid, and the slit is provided so as not to pass through any intersection of the grid. However, even if the slit width is 20 μm, the outline of the sensor part is visually recognized. In addition, even if the slit does not pass through any intersection of the grid, the visibility cannot be sufficiently reduced. Further, since a projected capacitive touchscreen generally has a structure in which a plurality of optically transparent electrodes are arranged in parallel on the same plane of a base material, the optically transparent electrodes in parallel to each other have a risk of short circuit.
Patent Literature 7 suggests a non-linear slit for a lower visibility than that of a linear slit, but this attempt also cannot sufficiently improve the visibility. Also, this method cannot solve the aforementioned problem of short circuit. In Patent Literature 8, the dummy part is formed of dots so that the sensor part and the dummy part have the same total light transmittance and as a result the same level of visibility. However, the metal pattern and the dots are different patterns, and therefore, an operator staring the touchscreen tends to recognize the difference, having a feeling of strangeness. Further, in cases of a metal pattern having a combination of minute dots and thin lines, due to halation that occurs in the exposure performed for metal pattern formation, a produced image may have a different size from designed values, resulting in unfavorable visibility.